This invention relates to measuring apparatus, and more particularly to apparatus for measuring the tension on a cable.
The present invention is of general application where it is desired to measure the degree of strain, or stretching, of a cable, chain, conduit, or similar object, due to a tension or stress on the object. It is particularly well suited for use in monitoring the tension on the drilling cable from which the drill string is suspended in a rig for drilling oil and gas wells, and thereby measuring the weight bearing on the drill bit.
In the rotary drilling of oil and gas wells, the drill bit is mounted at the lower end of a column of drill pipe, the drill string. The drill string is rotated by means of a square section of pipe at the top of the drill string, the "kelly", which passes at the surface through a square opening in a rotary table, which in turn is rotated by a suitable engine. The entire drill string is raised and lowered by means of a cable and associated tackle, driven by draw works.
In order to ensure straight and efficient cutting by the drill bit and avoid damage to the bit or pipe column, it is desirable to control the pressure on the drill bit by suspending part of the weight of the drill string from the drilling cable. The operator detects changes in the weight on a weight indicator, and operates the draw works to take up an appropriate portion of the weight.
Attempts have been made heretofore to provide such a weight indicator. One common type of prior apparatus for this purpose kinks or deflects a portion of the deadline, which is the anchored end of the drilling cable, so that the tension on the cable tends to straighten the misaligned portion. The straightening force is sensed by a diaphragm and transmitted hydraulically to the location of the operator.
In another form of sensor, yokes are secured to the cable and then drawn together by connectors running between arms attached to the yokes, thus removing the entire load from the section of the cable between the yokes and causing a kink in the cable. The tension is sensed by mechanical or hydraulic means forming part of the connectors.
These devices have the disadvantage, however, of undue complexity, and fail to take advantage of modern electrical technology, such as strain gauges. Further, when a kink or deflection is put in the deadline, an unnecessary stress is placed on the material of the cable, hastening metal fatigue and ultimate breakage. Also, the supporting arms of the load-bearing connectors of some of the devices are subject to substantial angular stresses, impairing their reliability. Finally, hydraulic devices are subject to inaccuracy caused by temperature changes.